The results of a one-year study of the effect of topic and situation on the speech behavior of fifth and sixth graders of Spanish-English background are reported. Two elementary schools in Los Angeles County were selected for the study. Students' language skills as evidenced in spontaneous speech obtained through sociolinguistic methods of observation and analysis were examined. These skills were compared with linguistic behavior in other situations. The differences between language behavior in the peer sociolinguistic interview sessions and in individual interviews are highlighted. It was found that morphology was only marginally affected by the situation, that syntax showed quite different patterning from morphology, and that length of residence showed a critical period during which speakers exhibited clear preference for either Spanish or English in peer interaction and in extended discourse. One major implication is that, while tests of morphology are useful for evaluating morphological development, they are not representative of functional speech behavior unless they allow for extended discourse. It is concluded that the concept of language proficiency, as applied to sudents on the basis of conventional language proficiency assessment instruments, should be distinguished from language abilities. (RW)